1. Field of the Invention
This invention relates to a gas sensor, and more particularly to a gas sensor having a heater to warm up a gas-sensitive element thereof.
2. Description of the Prior Art
In general, there are two types of gas sensors, i.e., contact-combustion type and semiconductor type.
The contact-combustion type gas sensor is made by depositing catalyst on the surface of a platimum wire. When a gas comes in contact with the catalyst, reaction (combustion) heat generated by such contact results in a temperature rise of the platinum wire. The change in the electric resistance of the platinum wire due to the temperature rise is measured to determine the presence or concentration of the gas.
The semiconductor type gas sensor uses those semiconductors whose electric resistance changes when a gas comes in contact with them, and such resistance change enables determination of the presence or concentration of the gas. Various oxide semiconductors, such as tin oxide (SnO.sub.2), zinc oxide (ZnO), titanium oxide (TiO.sub.2), and cobalt oxide (CoO), are used to make gas sensors for various purposes while selecting suitable operating temperature and kind of catalyst to be carried thereby for each specific purpose. Examples of the oxide smiconductor gas sensors are propane gas sensors, carbon monoxide (CO) sensors, oxygen (O.sub.2) sensors, and humidity sensors.
Built-in heaters are often provided in such gas sensors for various purposes, such as for burning off contaminants precipitated on their surface, for preventing deposition of contaminants, and for enhancing the acitivity of their gas-sensing elements.
To detect any change in the resistance of the gas-sensing elements, it has been practiced as a convenient method to apply a known constant voltage thereto from the outside. With the constant voltage, the resistance change is reflected in a change of current therethrough, and the current change may be detected as a voltage change across a comparison resistor through which the current flows.
The voltage for the resistance change detection is not necessarily the same as that for the heater, so that gas sensors of the prior art have a shortcoming in that two separate power sources are necessary, one for the heaters and one for the resistance change detection. The need of two power sources tends to increase the manufacturing cost of the gas sensor.
For instance, oxygen sensors are used to keep automobile engine exhaust gas clean. More specifically, an oxygen sensor is used to determine whether the air-fuel-ratio in the composition of automobile engine exhaust gas is above or below the theoretical optimal value. A typical oxygen detector for this purpose detects resistance change in an oxide semiconductor element, such as an element made of titanium oxide (TiO.sub.2), cobalt oxide (CoO), and tin oxide (SnO.sub.2). The sensor output (in the form of electric signal) is fed back to an engine control system so as to keep the engine exhaust gas composition in a narrow range around the theoretical optimal air-fuel-ratio which range is suitable for efficient action of ternary or other catalyst therein. In this case, it has been considered preferable to provide a heater in the oxygen sensor because the sensor operation is unstable for low temperature exhaust gas below 400.degree. C. such as that from cold engine.
As to the power source for such heater, a car battery rated at 6 V or 12 V is convenient and desirable from the standpoint of heater design. On the other hand, as to the power source for the oxygen sensor to detect the resistance change of its gas-sensitive element, a lower voltage of 1-5 V is desirable from the standpoint of gas sensor durability. Thus, two kinds of power sources are necessary in this case.
The conventional gas-sensitive element which detects resistance change thereof has another shortcoming in that the resistance value of the gas-sensitive element itself is affected by temperature. Thus, the measured value of the resistance change involves both a temperature depending portion and a gas concentration depending portion. When such gas-sensitive element is used over a wide temperature range, its accuracy of detection is inevitably low. To minimize the temperature influence, a temperature compensating element may be used in conjunction with the gas-sensitive element, but such compensating element causes an increase in the gas sensor cost.